Technological development and increased demand for mobile devices have led to rapid increase in the demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries having high energy density, high operating voltage, long cycle span and low self-discharge rate are commercially available and widely used.
In addition, increased interest in environmental issues has recently brought about a great deal of research associated with electric vehicles (EV) and hybrid electric vehicles (HEV) as alternatives to vehicles using fossil fuels, such as gasoline vehicles and diesel vehicles, which are a main cause of air pollution. Such electric vehicles generally use nickel-metal hydride (Ni-MH) secondary batteries as power sources. However, a great deal of study associated with use of lithium secondary batteries having high energy density, high discharge voltage and stable output is currently underway and some are commercially available.
Lithium secondary batteries may be classified into lithium-ion batteries containing liquid electrolytes per se, lithium-ion polymer batteries containing liquid electrolytes in a gel form, and lithium polymer batteries containing solid electrolytes, depending upon the type of electrolyte employed. Particularly, use of lithium-ion polymer or gel polymer batteries is on the rise due to various advantages thereof such as high safety owing to a low probability of fluid leakage, as compared to liquid electrolyte batteries, and the possibility of achieving very thin and lightweight batteries.
A lithium-ion battery is manufactured by impregnating a liquid electrolyte containing a lithium salt into an electrode assembly that includes a cathode and an anode, each being formed by applying an active material to a current collector, with a porous separator interposed between the cathode and anode.
Methods for fabricating a lithium-ion polymer battery are divided into a fabrication method of a non-crosslinked polymer battery and a fabrication method of a directly-crosslinked polymer battery, depending upon the type of a matrix material for electrolyte impregnation. Acrylate- and methacrylate-based materials having high radical polymerization reactivity and ether-based materials having high electrical conductivity are typically used as the polymer matrix materials. In particular, in directly-crosslinked polymer battery fabrication, a battery is fabricated by placing a jelly-roll type or stack type electrode assembly composed of electrode plates and a porous separator in a pouch, injecting a thermally polymerizable polyethylene oxide (PEO)-based monomer or oligomer crosslinking agent and an electrolyte composition into the pouch, and thermally curing the injected materials. Manufacture of batteries in this manner is advantageous in that electrode plates and separators of conventional lithium-ion batteries are used without change. However, directly-crosslinked polymer battery fabrication has problems in that a crosslinking agent is not completely cured and remains in the electrolyte, increasing viscosity. This makes uniform impregnation difficult, thereby greatly degrading battery properties.
A carbon-based material is typically used as an anode active material for lithium secondary batteries. However, the carbon-based material has a low potential of 0V relative to lithium and thus reduces the electrolyte, generating gases. Lithium titanium oxide (LTO) having a relatively high potential is also used as an anode active material for lithium secondary batteries to solve these problems.
However, when LTO is used as an anode active material, the LTO acts as a catalyst, generating a large amount of hydrogen gas during activation and charge/discharge processes, which causes a reduction in secondary battery safety.
Thus, there is a great need to provide a technology that secures battery safety by solving the above problems while maintaining overall battery performance.